Non-aqueous electrolyte secondary batteries represented by lithium-ion secondary batteries have high energy density and high voltage, and do not cause the memory effect (a phenomenon in which a battery gradually loses its capacity when charged before it is completely discharged) on charge or discharge. Owing to these advantageous features, non-aqueous electrolyte secondary batteries are used in a variety of fields, such as the field of mobile devices, the field of laptop personal computers, and the field of portable devices.
In general, the above non-aqueous electrolyte secondary battery is composed of a cathode plate, an anode plate, a separator, and a non-aqueous electrolyte. For the cathode plate, an electrode plate having an electrode active material layer formed by a particulate cathode active material that is fixed to the surface of a current collector made of metal foil or the like is usually used. For the anode plate is usually used an electrode plate having an electrode active material layer formed by a particulate anode active material that is fixed to the surface of a current collector made from copper, aluminum, or the like.
A conventional method for producing an electrode plate that serves as the above cathode or anode plate is as follows. An electrode active material layer-forming composition in the form of a slurry is first prepared by kneading and/or dispersing, in a solvent, a particulate electrode active material that is a particulate cathode or anode active material, a resin binder, and a conductive material (provided that when electrode performance can be fully obtained without a conductive material, e.g., in the case where the particulate anode active material also has electrical conductivity, the conductive material may not be used), and, if necessary, other materials. The electrode active material layer-forming composition is applied to the surface of a current collector and then dried, and the coating film thus formed on the current collector is pressed, thereby obtaining an electrode plate having an electrode active material layer (e.g., JP 2006-310010A and JP2006-107750A).
The particulate electrode active material which is used in the electrode active material layer-forming composition is a particulate metallic compound dispersible in the composition. The particulate metallic compound itself cannot fix well to the surface of a current collector even if pressed after it has been applied to the current collector surface and then dried, and easily peels off the current collector. In order to overcome this drawback, a resin binder is added to the electrode active material layer-forming composition, and by means of the resin binder, the particulate electrode active material is fixed to a current collector to form an electrode active material layer. Thus, a resin binder has been considered to be a substantially essential ingredient of the electrode active material layer-forming composition.
In recent years, the development of lithium-ion secondary batteries for use in the fields of electric vehicles, hybrid vehicles, power tools, etc. that are needed to have high output and input characteristics has been advanced. Further, even secondary batteries for use in relatively-small-sized devices, such as mobile phones, are expected to have improved output and input characteristics, since such devices tend to be provided with a larger number of functions. In order to realize improvement of secondary batteries in output and input characteristics, it is necessary to decrease the impedance of the secondary batteries. This is because secondary batteries having high impedance suffer some problems; e.g., they cannot make the best use of their capacities on high-speed charging and discharging.
In order to decrease the impedance of a secondary battery, decreasing the impedance of the electrode plates of the secondary battery is effective, and increasing the electrode areas by making the electrode active material layers in the electrode plates thinner has been known as a means for decreasing the impedance of the battery. Further, since non-aqueous electrolytes for use in lithium-ion secondary batteries generally have higher resistivity than aqueous electrolytes, there has been discussed, from the beginning of development of lithium-ion secondary batteries, an embodiment using thinner electrode plates with larger electrode areas and a smaller electrode gap between the cathode and anode plates than those in other batteries such as lead accumulators.
However, when the presence of ingredients other than the particulate active material in the electrode active material layer is also taken into account, it is impossible to make the electrode active material layer thinner without limitation. Practically, the lower limit of the thickness of the electrode active material layer has been about several tens micrometers.
Another effective approach to improvement of electrode plates in high output and input characteristics is the use of a particulate active material with a smaller particle diameter. The use of a particulate active material with a smaller particle diameter can make the total surface area of the particulate electrode active material in the electrode active material layer larger, and moreover, it can make the distance of movement, in one particle of the electrode active material, of lithium ion that intercalates in and deintercalates from the particle of the electrode active material shorter. Consequently, the behavior of lithium ion becomes smoother, which leads to improvement in output and input characteristics.
Practically, however, the viscosity of the electrode active material layer-forming composition tends to increase as the particle diameter of the particles of the active material decreases. This tendency was significantly observed especially when a particulate active material with a particle diameter of 11 μm or less, or with a particle diameter much smaller than this, was used. For this reason, the practicable particle diameter of the particulate active material is substantially limited, which has been disadvantageous to the above attempt to make the electrode active material layer thinner.
Besides the above-described goal of attaining high output and input characteristics, improving cycle characteristics for the purpose of extending battery life is another important goal for non-aqueous electrolyte secondary batteries.